The present invention relates to a vehicle braking system with a pneumatic brake booster which includes an input member being actuated by a vehicle operator and an output member (push rod), as well as a control housing that carries a movable wall and accommodates a reaction plate at least in part, wherein there is further provision of means for detecting the point of maximum boosting of the pneumatic brake booster that can be used for the actuation of another brake force boosting device.
A braking system of this type is disclosed in EP 754 607 A2 and comprises effective means for detecting the point of maximum boosting of the pneumatic brake booster as well as an additional boosting device that can be activated by virtue of the means of detection when the point of maximum boosting of the pneumatic brake booster is reached. The means for detecting the point of maximum boosting comprises a pneumatic pressure switch and an electric (contact) switch in the area of a reaction plate, which latter switch includes two opposed contacts on the reaction plate and a conducting path on a valve piston. An electric evaluation unit detects the point of maximum boosting in the case that both switches are actuated, i.e., electrically closed.
The known method is intricate because there are two switches and the evaluation of the signals of the switches. Not only is increased effort and structure incurred thereby with respect to the calibration of the two switches, but there are also additional expenditures in hardware.
Also, each cable must be led out of the interior of the brake booster in a fluid-tight manner and conducted to an electric control unit. The implementation of this system further necessitates evaluation software that takes two switches into account. One single switch in the area of the reaction plate will not solve this task because the actuation of the brake booster causes a contact between the valve piston and the reaction plate, without the point of maximum boosting being automatically reached.
One single pneumatic switch or sensor for sensing the pneumatic pressure in the chamber of variable pressure (working chamber) is unfavorable because in the event of the ventilation channel being clogged, for example, the boosting force will develop only very slowly or not at all. Because the pneumatic switch is consequently not actuated, the defect will not be detected although the vehicle operator requests an increased boosting force. Therefore, no signal can be produced for the activation of an additional auxiliary force source.
An object of the present invention is to provide a simplified and, additionally, improved brake system with a detection of the point of maximum boosting which outputs a corresponding signal, even in cases in which the pneumatic brake booster does not supply the requested boosting force.
This object is achieved, along with the features of the characterizing portion of patent claim 1, in that a switching or sensor means is provided which senses a maximum approach above a switching threshold between a component of the brake booster representative of the input force (Fe), especially an input member, and a component of the brake booster representative of the boosting force (Fv), especially a movable wall or a component connected thereto, and wherein there is provision of means to detect the hold time so that the time interval during which the maximum approach which occurs between the two components that are movable relative to one another can be determined, and a signal reporting the point of maximum boosting will only be output after expiry of a predefined time interval.
The approach between the input member (valve piston) and a control housing that carries the movable wall is sensed in another embodiment of the present invention.
Further, it is favorable to arrange for a slot in the area between the valve piston and the control housing, the slot decreasing with increasing boosting force of the brake booster. It is advisable to take a completely used-up or at least greatly reduced slot (an abutment between valve piston and control housing in the area of the switch-sensor) into account for sensing the point of maximum boosting. Accordingly, the switch/sensor with its actuating element must be integrated into the control housing or the valve piston.
It will be appreciated that the electric signals of the switch/sensor are sent to an electric control unit by way of a fluid-tight cable connection. Preferably, the control unit concerns an electric control unit of an ABS control system, e.g. with driving dynamics control functions, or an electric control unit of an independently operable, preferably electromagnetically operable, brake booster. Control units of this type are already provided in almost all modern vehicles so that the expenditure in hardware for the control unit is further reduced.
It is also favorable to employ a micro switch with especially small dimensions to save mounting space.
According to the characterizing portion of an alternative independent patent claim, the object of the present invention is achieved by the provision of a switching or sensor means which senses the maximum boosting force on the basis of a maximum deformation of a reaction element that is indirectly arranged between an output element (push rod) and a movable wall in that the maximum approach between the output member representative of the output force (Fa) and the movable wall representative of the booster force (Fv) is sensed.
According to the characterizing portion of another independent patent claim, the object of the present invention is achieved in that a switching or sensor means is provided which senses the maximum booster force (Fv) on the basis of a maximum deformation of a reaction element that is indirectly arranged between output element and input element in that the maximum approach between an output member representative of the output force (Fa) and an input member representative of the input force (Fe) is sensed.
A major advantage of the present invention includes that the point of maximum boosting is determined directly in the range of the differential forces to be compared, without the need to arrange for two switches or sensors and, more particularly, without the necessity of a pneumatic pressure sensor.
Further advantageous embodiments of the present invention can be taken from the sub claims in connection with the description and the accompanying drawings.